In casting processing or resin injection molding and the like using solidification of a melted material, air or other gas is entrained when filling the die cavity with molten metal resulting in gas defects, while the entrained minute amounts of gas and the like also contribute to the occurrence of shrinkage cavity defects. In particular, since oxide films easily form on the surface of the molten metal in the case of Al alloys or Mg alloys and the like, these films are easily entrained during the mold filling and causes inferior mechanical and chemical properties of the cast product, thereby making it desirable to prevent the formation of oxide films on the surface as well as different parts of the surface from colliding with each other.
Various casting methods have been developed in the past to accommodate such problems. For example, in a low pressure casting method, since molten metal is gently pushed up from the lower portion of a die to fill a die cavity, if the pressure acting on the surface of the molten metal in a holding furnace or a melting furnace can be suitably controlled over time, casting can be carried out without entraining gas and without causing different parts of the melt surface to collide with each other. However, this pressure control is not easy, and in the case of a shape such that the molten metal drops down in the die cavity in particular, there is susceptibility to entrainment of gas and oxide films on the melt surface. In addition, it is necessary to realize directional solidification such that solidification of the connection between the mold and feeding tube (gate) occurs at the final solidified location, thereby resulting in low productivity. In addition, it is difficult to pressurize unsolidified melt in the die cavity to prevent shrinkage defects.
A vacuum suction method has been applied practically for filling a die cavity while preventing oxidation of the melt surface by reducing pressure inside the die cavity instead of pressurizing the holding furnace to feed the molten metal. However, in this method, it is difficult to realize high vacuum due to movement of the surface, thus making it impossible to adequately prevent oxidation of the surface. Moreover, although gas is entrained into the die cavity unless the rate of depressurization is suitably controlled over time depending on the shape and dimensions of the die cavity, this control is not easy. In addition, productivity is also not satisfactory similar to low pressure casting methods of the prior art.
Although die casting methods have good productivity, in the case of a cold chamber system, the plunger sleeve is unable to be filled with molten metal resulting in the entrainment of gas and oxide film. Cold flakes formed as a result of the molten metal contacting the plunger sleeve are entrained and cause defects. Although a vacuum die casting method has been developed that reduces pressure in a short period of time after blocking the pouring hole with the tip of the plunger in order to prevent this gas entrainment, it is not easy to depressurize to a high vacuum in a short period of time similar to the previously described vacuum suction method. Consequently, although there are high vacuum die casting methods for realizing high vacuum by depressurizing both the die cavity and the holding furnace, these methods have not proliferated that much due to high equipment and maintenance costs. In addition, although ultra-high-speed injection die casting methods have been developed consisting of using a vacuum die casting method while increasing the gate speed beyond ordinary gate speeds, these methods are associated with increased equipment, maintenance and operating costs while also consuming large amounts of energy. Moreover, they also subject the die to a large load resulting in increased die costs and decreased dimensional accuracy.
Although squeeze casting methods result in little gas entrainment since gas in the plunger sleeve is able to be initially discharged, preventing the gas entrainment is not easy for the same reasons as in low pressure casting methods. In addition, the equipment is excessively high resulting in high building costs. Moreover, die costs are high due to the need to apply high pressure, thus resulting in a low degree of proliferation of this method.
With respect to die casting methods for Al alloy and the like, although the PF method attempts to demonstrate effects similar to a vacuum by filling the die cavity with oxygen and reacting with an alloy injected in the form of liquid droplets to form an oxide, it is not easy to completely remove gas.
In addition, although a hot chamber type of die casting method is able to prevent the formation of cold flakes in the plunger sleeve, it is difficult to prevent the gas entrainment for the same reasons as in the previously described cold chamber type of die casting methods, while durability of the plunger sleeve presents an additional problem with respect to Al alloys and the like.
In this manner, although various casting methods have been developed in the prior art, there has yet to be developed an ideal casting method that is free of entrainment of gas and oxide film of the surface of molten metal, has low equipment and maintenance costs, uses little energy, has small equipment dimensions and has good productivity.